1. Field of the Invention
My invention relates generally to the field of the float-zone fabrication of semiconductors and to the controlled solidification of other solids, such as alloys and glasses, in float zones and, more particularly, to a method and apparatus for suppressing or controlling the various convections in the float-zone which are induced by various causes, for example, the socalled Marangoni convection driven by the shear forces at the free surface and which are induced by the variation of surface tension in the free surface of the float zone due to the variation in temperature there, and also sometimes due to another substance, called a surfactant, present at the free surface in varying concentration.
2. Description of the Prior Art
In float-zone processing on earth where there is a gravitational force of 1 g, there may be three flows or convections which occur in the float zone: Thermal convection, solutal convection, and Marangoni convection; these convections combine to produce a resultant convection. On earth, the dominant component of convection is often, but not always, the thermal convection which is sometimes much larger than the Marangoni convection. However, in a near zero-gravity environment, such as in outer space or in orbital flight, the thermal and solutal convections are virtually eliminated, leaving only the Marangoni convection which is produced by the surface-tension gradient. Since conducting float-zone processes in space-flight is very expensive, such can be justified only if the material produced in the zero-gravity environment is far superior to that produced on earth. Consequently, in space-flight, it becomes important to eliminate, or to control to a specified level, the Marangoni convection which is relatively less important on earth, thereby, in the case of growing doped silicon crystals, for example, producing uniform distribution on a microscopic scale of the dopant to produce corresponding uniform resistivity on a microscopic scale, or to produce desired energy states, providing more stable and uniform growth of the resolidification interface, and producing crystals which are larger with better quality. In the case of other materials, such as alloys and glasses, the zero-gravity environment will produce better alignment of the microstructure in alloys, such as the super alloys used in turbine blades, and/or better uniformity of the microstructure, e.g. more uniform solution of one component in the other, thereby providing better mechanical, optical and/or electromagnetic properties in alloys, glasses etc. In some applications it is most desirable to eliminate the convection. In certain other cases, it may be preferable to control the convection to a specified optimal level in a particular region, as, for example, to inhibit dendrite growth on the resolidification surface. This invention applies to both cases.
There have been discussed in the literature the thermal, solutal, and Marangoni convections occurring in float zones and also, specifically, the Marangoni convection occurring in a zero gravity environment. (See "The Marangoni Effects", Scriven et al, Nature, July 16, 1960, Volume 187, pages 186-187 and "Studies of Floating Liquid Zones in Simulated Zero Gravity", Carruthers et al; Applied Physics, February, 1972, Volume 43, No. 2, pages 436-445). Furthermore, U.S. Pat. No. 4,046,617 discloses a method of refining or growing bulk single crystals in a zero gravity environment, but does not recognize or solve the problem of Marangoni convection. U.S. Pat. No. 3,086,856 discloses a float-zone process on earth utilizing a magnetic field or streams of gas to support the float zone against the gravitational force on earth. No mention is made of the fluid convections. U.S. Pat. No. 3,976,536 discloses a float-zone process conducted on earth and employing a gas stream for purposely inducing more rapid motion and turbulence in the float zone, allegedly to obtain constant dopant distribution in a semiconductor to produce constant resistivity on a macroscopic scale; recent research, however, indicates that uniform resistivities on the desired microscopic scale are best obtained by relying only on diffusion with as little convection as possible within the float zone. Furthermore, the increased convection will produce crystals with more imperfections.